Generally, in carrots or bellflower roots, microorganisms are found to exist at a density of 107 CFU/g therein, out of which pore-forming thermostable microorganisms, resistant to heat, exist at a density of 101 CFU/g or more. Also, parboiled green bean sprouts or asters have microorganisms at a density of 102˜105 CFU/g, with thermally resistant microorganisms at a density of 101 CFU/g. In the case of dried brackens, thermally resistant microorganisms live at a density of 102 CFU/g.
Thus, when various greens are cooked in a household, microorganisms are impossible to control completely, so that the greens, although cooked, are poor in preservability. Microorganisms may grow in the greens even while they are stored in a refrigerator after being cooked. Parboiled greens and dishes made therefrom also suffer from the same problems.
Microorganism control, which is indispensable for the storage of greens for a long period of time, is difficult to conventionally apply to greens. Retort sterilization, one of the most typical methods for inhibiting microbial growth, which is achieved by autoclaving at 121° C. for 4 min, is not applied to greens because it makes greens too frail. A pH reduction to 4.2 or below can control microorganisms, but makes the greens too acidic to eat.
Methods have been suggested for improving the freshness and preservability of greens by applying germanium compounds thereto (Korean Patent No. 10-0050101-0000) or by treating with chitosan and drying (Korean Pat. Laid-Open Publication No. 10-2003-0055223). These methods, however, are not adapted for ready-to-eat foods, and also suffer from the same problems as mentioned previously when being cooked. Due to the negative opinions of consumers on irradiated foods, irradiation with gamma rays (Korean Pat. No. 10-0045869-0000) can be carried out with only a low commercial success rate.